AD7294BCPZRL datasheet(46/48 Pages) AD

46 / 48 page

Data Sheet

AD7294

Rev. I | Page 45 of 47

LAYOUT AND CONFIGURATION

POWER SUPPLY BYPASSING AND GROUNDING

For optimum performance, carefully consider the power supply

and ground return layout on any PCB where the AD7294 is

used. The PCB containing the AD7294 should have separate

analog and digital sections, each having its own area of the

board. The AD7294 should be located in the analog section

on any PCB.

Decouple the power supply to the AD7294 to ground with

10 µF and 0.1 µF capacitors. Place the capacitors as physically

close as possible to the device, with the 0.1 µF capacitor ideally

right up against the device. It is important that the 0.1 µF

capacitor have low effective series resistance (ESR) and low

effective series inductance (ESL); common ceramic types of

capacitors are suitable. The 0.1 µF capacitor provides a low

impedance path to ground for high frequencies caused by

transient currents due to internal logic switching. The 10 µF

capacitors are the tantalum bead type.

The power supply line should have as large a trace as possible to

provide a low impedance path and reduce glitch effects on the

supply line. Shield clocks and other components with fast

switching digital signals from other parts of the board by a

digital ground. Avoid crossover of digital and analog signals,

if possible. When traces cross on opposite sides of the board,

ensure that they run at right angles to each other to reduce

feedthrough effects on the board. The best board layout

technique is the microstrip technique where the component

side of the board is dedicated to the ground plane only and the

signal traces are placed on the solder side; however, this is not

always possible with a 2-layer board.

Layout Considerations for External Temperature Sensors

Power amplifier boards can be electrically noisy environments.

Take care to protect the analog inputs from noise, particularly

when measuring the very small voltages from a remote diode

sensor.

Take the following precautions:

•

Place the remote sensing diode as close as possible to

the AD7294. If the worst noise sources are avoided, this

distance can be 4 inches to 8 inches.

•

Route the D+ and D− tracks close together, in parallel,

with grounded guard tracks on each side. Provide a ground

plane under the tracks, if possible.

•

Use wide tracks to minimize inductance and reduce noise

pickup. A 10 mil track minimum width and spacing is

recommended, as shown in Figure 61.

AGND

D1+

D1–

AGND

0.25mm

Figure 61. Arrangement of Signal Tracks

•

Try to minimize the number of copper/solder joints

because they can cause thermocouple effects. Where

copper/solder joints are used, make sure that they are

in both the Dx+ and Dx− path and are at the same

temperature.

•

Place a 10 pF capacitor between the base and emitter of the

discrete diode, as close as possible to the diode.

•

If the distance to the remote sensor is more than 20 cm, the

use of twisted-pair cable is recommended.

Because the measurement technique uses switched current

sources, excessive cable and/or filter capacitance can affect the

measurement. When using long cables, the filter capacitor can

be reduced or removed.

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